Dual volute turbocharger

ABSTRACT

A two-volute low pressure turbocharger is provided with a VGT mechanism in one turbine volute only.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/174,391, “Multi-Stage Turbocharging System with Efficient Bypass,”filed Jul. 1, 2005, now U.S. Pat. No. 7,644,585 which claims priorityfrom U.S. Provisional Patent Application No. 60/605,898, filed Aug. 31,2004. These above-mentioned applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to turbocharging systems for internal combustionengine systems.

2. Description of the Related Art

Turbocharging systems, such as for use with internal combustion engines,are well-known in the art. A turbocharger comprises an exhaust gasturbine coupled to a gas intake charge compressor. The turbine operatesby receiving exhaust gas from an internal combustion engine andconverting a portion of the energy in that exhaust gas stream intomechanical energy by passing the exhaust stream over blades of a turbinewheel, and thereby causing the turbine wheel to rotate. This rotationalforce is then utilized by a compressor, coupled by a shaft to theturbine wheel, to compress a quantity of air to a pressure higher thanthe surrounding atmosphere, which then provides an increased amount ofair available to be drawn into the internal combustion engine cylindersduring the engine's intake stroke. The additional compressed air (boost)taken into the cylinders can allow more fuel to be burned within thecylinder, and thereby offers the opportunity to increase the engine'spower output.

In a turbocharged internal combustion engine system, the wide range ofspeed and power output levels at which the internal combustion enginemay operate presents challenges for designing an appropriately matchedturbocharging system with good mechanical efficiency for working withthe engine. For example, while smaller turbochargers provide boostquickly and more efficiently at lower engine speeds, largerturbochargers provide boost more effectively at higher engine speeds.Because of the relatively narrow flow range over which a turbochargeroperates efficiently, relative to the broader flow range generated byinternal combustion engines, it is known in the prior art (e.g., incases of high boost need), to provide a multi-stage turbochargingsystem, involving both a smaller (i.e. “high pressure”) turbocharger anda larger (i.e. “low pressure”) turbocharger, wherein the smaller highpressure turbocharger operates at lower engine speeds and the larger lowpressure turbocharger takes over at higher engine speeds. It has beenfound valuable to switch between the two turbocharging stages throughuse of a bypass system to divert exhaust gas flow around the higherpressure turbocharger to the lower pressure turbocharger as needed.

As a result, bypassing exhaust flow around a turbine gas expander isalso well-known in the art. Typically, turbine bypass systems are usedin the prior art primarily to regulate system pressure across the higherstage turbine wheel, and can be operated by selectively bleeding off aportion of the upstream exhaust gas over a pressure drop through abypass channel when backpressure caused by the turbine's operationcauses the system pressure upstream of the turbine to exceed desiredlevels. Bleeding of the exhaust gas through the bypass channel isgenerally controlled by a small regulating valve (called a “wastegate”)in the exhaust piping channel around the turbine. A typical wastegatevalve operates somewhat like a trap door, opening a port from the higherpressure turbine inlet to a lower pressure area by diverting a portionof the exhaust flow through a bypass channel around the turbine, withthe bypassed exhaust flow naturally expanding over the pressure drop inthe bypass channel and then reuniting with the remaining exhaust flowdownstream of the bypassed turbine.

OBJECT OF THE INVENTION

An object of the present invention is to provide a more efficientmulti-stage (i.e., with two or more stages) turbocharging system forinternal combustion engine systems.

In furtherance of the object of this invention, it has been recognizedthat prior art wastegate and bypass mechanisms are a source ofunnecessary loss of useful energy in prior art multi-stage turbochargingsystems. Therefore, a further object of the present invention is toprovide an efficient means for preserving, capturing, utilizing, and/orreducing the amount of energy otherwise lost in bypassing between stagesin multi-stage turbocharging systems, in order to further improve theefficiency of internal combustion engine systems utilizing multi-stageturbocharging systems.

SUMMARY OF THE INVENTION

The present invention reduces the unrecovered loss of exhaust gas energythat otherwise occurs in bypassing exhaust flow from one stage toanother in a conventional multi-stage turbocharging system. Thepreferred method of preserving such exhaust energy is through convertinga portion of the exhaust energy of the bypassed flow from pressure tovelocity by passing the bypassed flow, while substantially still at thehigher exhaust energy level present upstream of the bypassed turbine,through a variable geometry valve/nozzle, turbine VGT vanes, or otherreduced cross-sectional area nozzle, and then not allowing theaccelerated flow to substantially lose that increased recoverablekinetic energy before reaching the subsequent stage's turbine wheel.This may be done, for example, through placing the variable geometryvalve or VGT vane outlet adjacent to the lower pressure turbine wheel'sblades (or sufficiently nearby such blades to avoid substantialdissipation of the increased acceleration/momentum effect), and at anappropriate incidence angle to the lower pressure turbine wheel'sblades. The increased momentum resulting from accelerating the flow maythen be imparted to the lower pressure turbine's wheel, and therebyallow converting the energy to a mechanical rotational force as is knownin the art. Alternative means and preferred turbocharging hardwareembodiments for efficiently preserving or capturing energy lost betweenstages in a multi-stage turbocharging system are also discussed. Thissystem may be utilized between stages with internal combustion engine orother multi-stage turbine systems encompassing three or four (or more)stage systems as well.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale.

FIG. 1 is a schematic diagram of an internal combustion engine systemwith a prior art multi-stage turbocharging system.

FIG. 2 is a schematic diagram of an internal combustion engine systemwith a first turbocharging and bypass arrangement of the presentinvention.

FIG. 3 is a schematic diagram of an internal combustion engine systemwith a second, alternative bypass arrangement of the present invention.

FIG. 4 is a more detailed view of the turbocharging and bypassarrangement of the system shown in FIG. 3.

FIG. 5 is a schematic diagram of an internal combustion engine systemwith another alternative turbocharging and bypass arrangement of thepresent invention.

FIG. 6A presents a preferred variable geometry valve/nozzle means foruse with a two-volute low pressure turbine, with the valve/nozzle meansbeing VGT vanes, shown in an open position, in a second volute of atwo-volute low pressure turbine.

FIG. 6B provides another view of the FIG. 6A preferred variable geometryvalve/nozzle means, but shown with the valve/nozzle means in a closedposition.

FIG. 6C presents a conventional rotating VGT vane, for use in a turbine.

FIG. 6D presents an alternative VGT vane with an articulating trailingedge.

FIGS. 7A and 7B are sectional views of a two-volute turbine, showing analternative variable geometry valve/nozzle means embodiment of thepresent invention for use in a second volute of a two-volute lowpressure turbine.

FIG. 8 is a sectional view of a two-volute turbine, showing a secondalternative variable geometry valve/nozzle means embodiment of thepresent invention for use in a second volute of a two-volute lowpressure turbine.

FIG. 9A illustrates the preferred internal combustion engine multi-stageturbocharging and bypass arrangement of the present invention, with apartially cut-away view of volute 53′ in the invention.

FIG. 9B illustrates the two volute turbine of the preferred embodimentof an internal combustion engine multi-stage turbocharging and bypassarrangement of the present invention.

FIG. 10 presents a cut-away view of a two-volute turbine in aside-by-side orientation, such as for use in the preferred embodiment ofFIGS. 9A and 9B of the invention.

FIG. 11 is a schematic view of another alternative embodiment of aninternal combustion engine system of the present invention, with twoturbines on a common shaft.

FIG. 12 is a more detailed view of the FIG. 11 two turbines on a commonshaft turbocharging and bypass arrangement of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an internal combustion engine system with a multi-stageturbocharging and bypass system from the prior art. Referring to FIG. 1,ambient air enters the system through intake line 11. The intake air mayoptionally be mixed with recirculated exhaust gas (EGR) to form acharge-air mixture. The ambient air or EGR/ambient air mixture(“charge-air”) mixture flows through and is compressed by a first-stagelow pressure air compressor 12.

After compression in compressor 12, the intake air may flow through asecond-stage high pressure air compressor 16 for further compression.Alternatively, the intake air may be diverted at port 13 to optionalbypass channel 14 and return to the intake line at port 17, as regulatedby the opening or closing of optional bypass valve 15.

Intake air then enters the intake manifold 18 and into combustionchambers 20 of engine 19 through conventional valves (not shown) in aconventional manner. Following combustion in the combustion chambers 20,the warm, pressurized exhaust gases leave the combustion chambers 20, ata first, higher, exhaust gas energy level, through conventional valves(not shown) in a conventional manner, and flow from engine 19 throughexhaust manifold 21 to exhaust line 28.

After leaving the exhaust manifold 21, exhaust gas in exhaust line 28may flow through a high pressure turbine gas expander 25. High pressureturbine gas expander 25 in exhaust line 28 is coupled to the highpressure air compressor 16 in the intake line 11 through shaft 29′, andtogether the combined expander and compressor device forms a highpressure turbocharger 30. Alternatively to flowing through high pressureturbine 25, a portion of the exhaust gas may be selectively diverted atport 22 to bypass channel 23 and return to the exhaust line at port 26,as regulated by opening or closing of port 22 through operation ofwastegate valve 24, which is operated (actively or passively) to open inresponse to system pressure buildup upstream of turbine 25. Wastegatevalve 24 may be located anywhere within bypass channel 23. It should benoted that even though the wastegated exhaust gas does not pass throughturbine expander 25, the pressure difference between the bypassedexhaust gas flow and the exhaust gas that has passed through turbineexpander 25 is lost to natural expansion and dissipation of anyincreased velocity of the bypassed exhaust gas, either in bypass channel23 or upon reuniting with the lower pressure exhaust flow in exhaustline 28 at port 26.

Downstream of turbine gas expander 25, the exhaust gas at this second,lower, exhaust gas energy level may then flow through low pressureturbine gas expander 27 for further expansion, and then exit the systemthrough exhaust line 28. It should also be noted for FIG. 1 that turbinegas expander 27 in exhaust line 28 is coupled to low pressure aircompressor 12 in intake line 11 through shaft 29, and together theexpander 27 and compressor 12 integrated device form a low pressureturbocharger 31.

FIG. 2 presents a first improvement on the prior art multi-stageturbocharged internal combustion engine system of FIG. 1, as oneembodiment operating in accordance with principles of the presentinvention. For ease of discussion in highlighting aspects of thisembodiment of the invention over the prior art, the embodiment of FIG. 2is presented herein as identical to FIG. 1 of the prior art in allrespects (i.e., with identical components, numeration, systemconfiguration and operation), except as hereafter described.

Referring to FIG. 2 in comparison to the FIG. 1 prior art, it will benoted that certain changes from the prior art have been made withrelation to the bypass system around high pressure turbine 25. Likevalve 24 of FIG. 1, valve 34 of FIG. 2 regulates (e.g. through apressure differential) the quantity of exhaust gas diverted from exhaustfine 28 through bypass means channel 33 to port 36. However, in FIG. 2,valve 34 and return port 36 are geometrically configured closer to, andat a more complementary angle for direction of the bypass flow towardthe inlet of turbine 27.

These changes in FIG. 2 are made in recognition that a portion of theenergy in the bypassed exhaust gas that is diverted through bypasschannel 33 is converted from pressure to kinetic energy (velocity) atvalve 34 by passing the bypassed flow through valve 34, with valve 34acting as a reduced cross-sectional area nozzle. Valve/nozzle 34therefore acts in FIG. 2 as a nozzle when in an open position, byproviding a reduced cross-sectional area flow path for the bypassedexhaust gas. As an example, valve/nozzle 34 may open to form a flow pathin the shape of a short tube with a taper or constriction (reducedcross-section) designed to speed up (and preferably also direct) theflow of exhaust gas. As will be known in the art, there are many knownalternative structures that may also perform this similar “nozzle”function of speeding up the flow of a gas or fluid, which are alsointended to be encompassed within this patent's use of the terms“nozzle” or “nozzle means” herein.

The accelerated flow exiting valve/nozzle 34 is there reunited atintersection point 36 with the flow in exhaust line 28 (or directly atan inlet to turbine 27), preferably in an orientation resulting in anoptimal combined direction for the exhaust flows just prior to, and atan appropriate incidence angle to, the turbine wheel blades of turbine27, as will be known in the art. The accelerated flow is thereconverted, combined with the exhaust flow in exhaust line 28, to amechanical rotational force by turbine 27. By locating port 36sufficiently near the turbine wheel blades of turbine 27, theaccelerated flow is not allowed to substantially dissipate energy beforereaching the turbine wheel of turbine 27 for work extraction. Regardingselection of acceptable distances between valve/nozzle 34 and theturbine wheel, it will be understood that the closer the distance willresult in greater recovery of energy, and that through experimentationthe distance can be increased until such point that the increase inrecovery of energy from the bypass acceleration is no longer measurablewith normal state of the art sensors and thus would no longer fallwithin the scope of this invention.

Thus, in FIG. 2, bypass means 33 and valve/nozzle 34 provide bypassingof pressurized exhaust gas from the engine around the high pressureturbine to an inlet of the lower pressure stage turbine in thisembodiment, by leaving the bypassed flow in a complementary flowingdirection with the main exhaust flow just prior to the turbine wheelblades of turbine 27, regardless of (and thus it is irrelevant for thisparticular embodiment) whether port 36 lies as a direct inlet to turbine27 or as a substantially equivalent return port to exhaust line 28 justprior to turbine 27.

FIG. 3 presents the same embodiment as FIG. 2, but illustrating that thelength of the bypass route 33 is irrelevant and may be substantiallyeliminated, if desired. In addition, for either FIG. 2 or 3, the bypassroute may optionally begin directly from exhaust manifold 21 instead ofexhaust line 28, if desired, such as is illustrated in FIG. 5 (discussedbelow).

FIG. 4 illustrates in more detail one embodiment of reuniting of theaccelerated bypass flow with the main exhaust flow prior to and at anappropriate incidence angle to the turbine wheel blades of turbine 27 asdiscussed for FIGS. 2 and 3 above. As shown in FIG. 4, bypass exhaustflow 49 in bypass route 33 passes through valve 34 in a reduced crosssection (nozzle) area of bypass route 33 and/or port 36, which producesan accelerated bypass exhaust flow 51. Other “nozzle means” foraccelerating the bypass exhaust flow may alternatively be used, as isknown in the art. Accelerated bypass exhaust flow 51 then combines withthe lower velocity main exhaust flow 50 in exhaust line 28 (or,alternatively, within the turbine 27 itself), forming combined exhaustflow 52. Combined exhaust flow 52 preferably shortly thereafter hits theturbine blades 48 at a desired angle to cause turbine wheel 47 to spin,as is known in the art. Note, however, that it is not necessary for thebypass flow to reunite with the main exhaust flow prior to impact withthe lower pressure turbine's wheel blades for the energy to berecovered.

FIG. 5 presents an alternative embodiment, with bypass route 43connected directly to exhaust manifold 21, instead of to exhaust line28. In this manner, for each of these embodiments, it will be understoodthat bypass means 43 may be shortened to be no more than a direct fluidconnection between exhaust manifold 21 and an inlet to low pressureturbine 27. In addition, returning to FIG. 5, in FIG. 5 a two-voluteturbine 27′ (e.g. FIG. 10) replaces turbine 27, with one volute 53 ofturbine 27′ receiving lower velocity and energy exhaust from exhaustline 28 downstream of high pressure turbine 25, and the other volute 53′of turbine 27′ receiving the higher energy and velocity (accelerated)bypassed exhaust directly from exhaust manifold 21 (without beingreunited with other exhaust gas prior to impact with the lower pressureturbine's wheel blades). Although the two-volute turbine 27′ is at timesreferred to herein as a “dual volute” turbine, such reference does notmean that volutes 53 and 53′ of turbine 27′ are necessarily the samesize. Two-volute turbines such as turbine 27′ are known in the art,although more commonly with volutes of the same size, such as for usewith divided exhaust manifolds. FIG. 10 presents a cut-away view of asample two-volute turbine 27′. It will also be understood that the flowsfrom the two volutes of turbine 27′ may be coordinated in various wayswith regard to the targeting of the respective flows toward the bladesof the turbine wheel, if desired.

For FIGS. 2 through 5 above, it has already been discussed that valve 34in the bypass route may function in the present invention as both (i) aregulating valve to control bypass flow, and (ii) as a nozzle thatconverts a portion of the exhaust energy of the bypassed flow frompressure to kinetic energy (in the form of increased velocity of thebypassed exhaust flow). Given the wide range of exhaust flows generatedin internal combustion engines that operate under wide ranges of enginespeed and load conditions, it is preferable with the present inventionto utilize a valve/nozzle means with variable geometry capability inaccelerating the bypassed exhaust flow, to extend the system's benefitsand effectiveness over a wider range of engine operation.

There are various structures that may be utilized to serve the functionsof valve/nozzle means 34 with the preferred variable geometrycapability. In FIGS. 6A and 6B, as a preferred embodiment for use with atwo-volute turbine 27′ (or also for single volute turbine 56 of the twoturbine arrangement in FIG. 12, discussed below), VGT vanes 54surrounding the turbine wheel 47 function as the valve/nozzle means.FIG. 6C presents a larger view of a conventional VGT vane 54.

FIG. 6A illustrates the vanes 54 in an open orientation, allowing andguiding passage of bypassed exhaust flow 49 to the turbine blades 48,and additionally acting as variable geometry nozzles in accelerating theexhaust flow 49 into the turbine blades 48. In contrast, FIG. 6B showsthe vanes 54 of FIG. 6A in a completely closed orientation (i.e. here,lined up “tail to nose” around the turbine wheel 47), thereby sealingand blocking any bypass exhaust flow through the volute 53′ to theturbine blades 48. In this manner, the position of the VGT vanes 54 canoperate fully as a regulating valve to open or shut off flow, anddictates the back pressure applied to the exhaust line 28 and/or exhaustmanifold 21 in the system, and thus also controls the pressure dropallowed for the main exhaust flow through the high pressure turbine 25.This consequently provides flow control through the alternative exhaustpaths, including proportional flow control, to extend the system'sbenefits and effectiveness over a wider range of engine speed and loadoperating conditions.

As an alternative to VGT vanes 54 for two-volute turbine 27′, FIGS. 7Aand 7B utilize a sliding plate mechanism 54′ in volute 53′ to performthe valve/nozzle function in regulating and accelerating the bypass flowin volute 53′ to turbine blades 48. Likewise, FIG. 8 utilizes a slidingmember/mechanism 54″, as shown for a two-volute double flow turbinehousing (wherein for this second example the two volutes areconcentrically disposed with respect to the circumference of the turbinewheel 47, as opposed to being side-by-side with respect to thecircumference of the turbine wheel 47).

FIGS. 9A and 9B now present the preferred embodiment of the multi-stageturbocharging system of the present invention. FIG. 9A is similar to theembodiment of FIG. 5, except as noted below. In the FIG. 9A preferredembodiment, valve/nozzle 34 is replaced by Variable Geometry Turbine(VGT) mechanism 54 in one volute, volute 53′, of two-volute turbine 27′.The two volutes 53 and 53′ are configured in a side-by-side orientationto each other with respect to their orientation around the circumferenceof the turbine wheel 47, as shown by the partial cut-away view in FIG.9A, and as also more clearly shown in FIG. 9B and in FIG. 10. VGTmechanism 54 is presented herein in FIGS. 9A and 9B as conventionalrotating adjustable vanes 54 (as also shown in FIGS. 6A-6C), but it willbe understood that other VGT and/or other nozzle mechanisms may also beequivalently employed (e.g. a sliding nozzle mechanism as used byCummins or in FIGS. 7A-7B and 8, or a vane with an articulating trailingedge (FIG. 6D), as a few examples) without departing from the scope ofthe invention.

As is known in the art, adjustable VGT vanes 54 act as nozzles tothrottle exhaust gas and use the resulting restriction to create anaccelerated, high velocity exhaust gas stream, and also to guide anddirect that exhaust gas stream into the turbine wheel blades 48 (e.g.,as is represented in FIG. 6A). Thus, in one embodiment, VGT mechanism 54comprises conventional VGT vanes, which are rotating vanes arranged in acircle in the turbine volute 53′, with the vanes able to rotateuniformly to form wider or narrower paths for the exhaust gas to theturbine blades 48. VGT mechanism(s) 54 are preferably placed closelyadjacent the turbine blades 48 such that the kinetic energy of thebypassed exhaust flow passing by such vanes is fully preserved and notlost prior to the bypassed exhaust flow hitting turbine blades 48 at theoptimal angle, as will be understood in the art. In contrast, secondvolute 53 of turbine 27′ is preferably a fixed volute without a VGTmechanism 54, but may optionally use VGT as well, if desired. The flowfrom both volutes 53 and 53′ target portion(s) of the turbine wheelblades 48 as desired, as for example shown in the sample embodiment ofFIG. 9B.

Further referring to FIG. 9A, high pressure turbine 25 (presented simplyin block form) is fluidly connected to exhaust manifold 21 by exhaustline 28. High pressure turbine 25 may optionally contain a VGTmechanism, if desired. Exhaust gas enters and leaves high pressureturbine 25 through an inlet and outlet in conventional manner (notshown), to continue in exhaust line 28 to volute 53 of low pressureturbine 27′, where it is further expanded. The further expanded exhaustgas then leaves low pressure turbine 27′ through an outlet inconventional manner (not shown), to continue in exhaust line 28 forexhaust gas recirculation or for release from the exhaust system. Bypassmeans/turbine inlet 43 of low pressure turbine 27′ is also fluidlyconnected to exhaust manifold 21, allowing high pressure exhaust gas tobypass high pressure turbine 25 to volute 53′ of low pressure turbine27′. VGT mechanism 54, as discussed above, here shown as adjustablerotating VGT vanes as one embodiment, acts in volute 53′ in place of avalve to regulate flow of bypassed exhaust gas flow 49 through volute53′, and also acts as a nozzle means to convert the exhaust energy inbypassed exhaust flow 49 to kinetic energy (velocity) to createaccelerated bypass exhaust flow 51, and to guide or direct theaccelerated bypass exhaust flow 51 to hit turbine blades 48 at anappropriate incidence angle (e.g., as shown in FIG. 6A) for spinning ofturbine wheel 47. The placement of the VGT mechanism 54 near the turbineblades 48 allows the kinetic energy and increase in momentum of thebypassed exhaust flow to be preserved (by not allowing deceleration andexpansion) for conversion to mechanical force at the turbine blades 48.The expanded exhaust gas from volute 53′ then leaves low pressureturbine 27′ through an outlet in conventional manner (not shown), tocontinue in exhaust line 28 for exhaust gas recirculation or for releasefrom the exhaust system, as discussed above.

FIG. 11 presents an alternative preferred embodiment of the enginesystem and turbocharging system of the present invention, similar toFIG. 5 and to FIGS. 9A and 9B, but comprising two low pressure turbines56 and 57 on a common shaft 29″ instead of a two-volute low pressureturbine 27′. Turbine 56 utilizes a VGT mechanism 54 in a configurationand manner similar to volute 53′ from FIGS. 5 and 9A-9B, and receivesbypass exhaust flow from bypass route 43 in one of the manners aspreviously described above. Turbine 57, on the other hand, preferablyutilizes a fixed geometry, and receives exhaust gas from exhaust line 28that has already passed through high pressure turbine 25, as alsopreviously described above. Each low pressure turbine 56 and 57 includesa separate turbine wheel arrangement (identified as turbine wheels 47and 47′, and blades 48 and 48′, as shown in FIG. 12), with the rotatingwheels 47 and 47′ connected by common rotating shaft 29″, which is alsopart of shaft 29, which connects the two turbines 56 and 57 tocompressor 12 (as shown in FIG. 11). The compressor 12, shaft 29 and29″, and two turbine arrangement 56 and 57 comprise turbocharger 31′ inthis embodiment.

After expansion in turbines 56 and 57 of FIG. 9, the exhaust gas flowsthat leave turbines 56 and 57 are thereafter combined downstream inexhaust line 28 (or within the two turbine turbocharger arrangementitself, in the alternative).

It will be understood from the foregoing that there are various otherembodiments that could also be formed to achieve the novel objectivesand methods of the inventions herein, and that such variations withequivalent functions and goals are also intended to fall within thescope of this patent. For example, the objectives of the inventionsherein. may apply to multi-stage turbines for gas or fluid flows inother applications than in conjunction with internal combustion engineturbocharging systems. This patent is therefore intended to be limitedsolely by the claims, in the manner allowed by law.

I claim:
 1. A two-volute turbine gas expander for a turbocharger,providing two paths for flow of a gas through the expander, comprising:a first volute within the turbine gas expander, with a first turbineinlet respectively fluidly coupled thereto, wherein the first volute hasa variable flow geometry; and a second volute within the turbine gasexpander, with a second turbine inlet respectively fluidly coupled tosaid second volute and to an outlet of a higher pressure turbine gasexpander to receive expanded gas from the higher pressure turbine gasexpander, wherein the second volute has a fixed flow geometry.
 2. Theturbine gas expander of claim 1, wherein VGT vanes are rotatably mountedwithin the first volute to provide the variable flow geometry for thevolute.
 3. The turbine gas expander of claim 2, wherein the VGT vaneshave an articulating trailing edge.
 4. The turbine gas expander of claim2, wherein the VGT vanes further provide proportional control of gasflow in the first volute.
 5. The turbine gas expander of claim 4,wherein the VGT vanes are configured to selectively open or shut off theflow of gas through the first volute.
 6. The turbine gas expander ofclaim 1, wherein a sliding mechanism is located within the first voluteto provide the variable flow geometry for the volute.
 7. The turbine gasexpander of claim 6, wherein the sliding mechanism is configured toselectively open or shut off the flow of gas through the first volute.8. The turbine gas expander of claim 7, wherein the sliding mechanismfurther provides proportional control of gas flow in the first volute.9. The turbine gas expander of claim 1, wherein the first volute is adifferent size than the second volute.
 10. The turbine gas expander ofclaim 1, wherein the first turbine inlet is configured to solely receivegas that bypasses the higher pressure turbine gas expander instead ofpassing through the higher pressure gas expander.
 11. The turbine gasexpander of claim 1 wherein the two paths provided by the first andsecond volutes are kept fully segregated upstream of a turbine wheel.12. The turbine gas expander of claim 1, further comprising: a turbinewheel located within the turbine gas expander, configured to rotate asgas flows through the expander over blades of the turbine wheel; and ashaft rotatably driven by the turbine wheel and coupled to a gas intakecharge compressor, wherein the shaft is configured to rotatably drivethe compressor.